Electroless deposition methods are now widely used in industry to deposit metals such as nickel and copper on a wide variety of insulating or conducting substrates. In general, deposition baths contain a salt of the metal to be deposited, a reducing agent capable of reducing the metal salt to metal, a chelating agent to control the reduction process and a pH-adjusting agent. Other substances such as stabilizers and wetting agents may also be present in the baths.
One of the most commonly used reducing agents in nickel-depositing baths is sodium hypophosphite. This reducing agent enables a desirably high rate of deposition to be achieved. However, in hypophosphite type baths, the metal that is deposited is alloyed with a certain percentage of phosphorus. For some purposes, such as the coating of plastics with nickel for decorative purposes, the percentage of phosphorus in the deposit is of no particular importance. But, if the nickel is being deposited on headers or heat sinks used in semiconductor devices, the percentage of phosphorus in the nickel is critical because it affects the solderability of the coated surface. Many commercial baths for depositing nickel and including a hypophosphite as the reducing agent have been found not to be suitable for plating on semiconductor devices because the nickel has variable solderability and other undesirable properties. This is due, in major part, to the phosphorus content of the surface layers of the deposited nickel-phosphorus alloy.
In the past, most electroless metal plating methods have been directed toward providing a continuous process including periodic replenishment of the bath with metal salt, reducing agent and any other ingredients, such as the pH-adjusting agent, that get used up in the process. Continuous plating is usually more economical in large volume production. However, in the electronics industry, reproducibility and uniformity of the plated surface throughout the production cycle is of primary importance. If the solderability or bondability of the plated surface keeps changing during assembly of semiconductor devices, the production results are often adversely affected to an extent that cannot be tolerated.
In order to provide greater uniformity and reproducibility of a nickel plated surface, the present method utilizes a process in which the metal content of the bath is allowed to become depleted to a low value at the end of each production run. The bath is discarded at the end of each production run and a new bath is made up for a new run. This produces a high level of consistency at a low cost in the initially used chemicals.
The present method also provides an initially high plating rate that holds for most of the plating run. Then the plating rate drops off rapidly, corresponding to an abrupt drop in pH, and an increase in the phosphorus content of the deposited coating to a controlled level. These results are achieved by providing buffers in the form of lactates and acetates (or other aliphatic carboxylic acid salts). The buffers provide an initially high plating rate, a relatively high initial acid pH and a relatively low initial phosphorus content in the deposit. The lactate also imparts good bath stability.
A glycolate is used to adjust the final pH of the bath. Too much glycolate prevents the pH from dropping to as low a level is desired. The correct amount of glycolate to use also depends on the concentration of hypophosphite and lactate. Under some conditions, the glycolate can be omitted entirely. A malate can also be used in place of the glycolate.